Allow specifying aliases for e.g. enum variants.

[ModProg]

I can see two implementations, either add an additional alias field to the musli attribute that takes a list of names, or extend the name field:

For serializing this doesn't change anything, but for parsing this would mean it needs to accept the primary as well as all aliases. AFAICT, this would only make sense when using names and not indexes.

#[musli(name = "primary", alias = "secondary")]
#[musli(name("primary", "secondary")]